1. Field of the Invention
This invention generally relates to a ferroelectric memory device and, more particularly, to a nondestructive readout-type ferroelectric memory device using a memory medium of a ferroelectric material.
2. Description of the Related Art
In general, a ferroelectric material has hysteresis characteristics. To utilize such hysteresis characteristics in a non-volatile ferroelectric memory device for storing data is widely known. The element structure of the ferroelectric memory device is divided broadly into the following two types:
One type is called a "simple matrix structure", wherein each intersection of strip electrodes which are formed on the upper and lower surfaces of a ferroelectric thin film such that they cross at right angles to one another is used as a memory cell. Since the simple matrix structure is a complicated structure, there is a limitation in producing a highly integrated ferroelectric memory device where the simple matrix structure is employed. Destructive readout, which uses a polarization-inverted current required to rewrite data into a selected cell, is performed to read data from each memory cell of the ferroelectric memory device of the simple matrix structure.
On the other hand, the other type structure is disclosed in U.S. Pat. No. 5,060,191 assigned to the same assignee as that of the present application. In this case, writing and reading operations are performed at low impedance by the use of the self-inversion phenomenon of a ferroelectric thin film, without adversely affecting non-selected cells. Here, the self-inversion phenomenon means a phenomenon in which the state of polarization of the ferroelectric thin film returns to its initial state when a pulse has been applied to the thin film from the outside.
However, the above-described conventional ferroelectric memory devices have the following problems:
In the case of the ferroelectric memory device which performs destructive readout, the ferroelectricity is deteriorated and accordingly the remanence is lowered, since polarization inversion is repeated. As a result, the memory device can hardly have a long life, and a complicated circuit is necessary to perform rewriting.
Further, in the case of the ferroelectric memory device disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2-154389, the publication does not disclose any specific mechanism or structure for realizing the self-inversion phenomenon of spontaneous polarization, although it is highly possible to use the ferroelectric memory device for realizing writing and reading in a simple matrix structure.